GEOMETRY 10-2 Proving Triangles Similar; AA, SSS, and SAS Warm Up Warm Up Lesson Presentation Lesson...

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GEOMETRY

10-2 Proving Triangles Similar; AA, SSS, and SAS

Warm UpWarm Up

Lesson PresentationLesson Presentation

Lesson QuizLesson Quiz

GEOMETRY

Warm UpSolve each proportion.

1. 2. 3.

4. If ∆QRS ~ ∆XYZ, identify the pairs of congruent angles and write 3 proportions using pairs of corresponding sides.

z = ±10 x = 8

Q X; R Y; S Z;

GEOMETRY

Prove certain triangles are similar by using AA, SSS, and SAS.

Use triangle similarity to solve problems.

Objectives

GEOMETRY

There are several ways to prove certain triangles are similar. The following postulate, as well as the SSS and SAS Similarity Theorems, will be used in proofs just as SSS, SAS, ASA, HL, and AAS were used to prove triangles congruent.

GEOMETRY

Example 1: Using the AA Similarity Postulate

Explain why the triangles are similar and write a similarity statement.

Since , B E by the Alternate Interior Angles Theorem. Also, A D by the Right Angle Congruence Theorem.

Therefore ∆ABC ~ ∆DEC by AA~.

GEOMETRY

TEACH! Example 1

Explain why the trianglesare similar and write asimilarity statement.

By the Triangle Sum Theorem, mC = 47°, so C F. B E by the Right Angle Congruence Theorem. Therefore, ∆ABC ~ ∆DEF by AA ~.

GEOMETRY

Example 2: Verifying Triangle Similarity

Verify that the triangles are similar.

∆PQR and ∆STU

Therefore ∆PQR ~ ∆STU by SSS ~.

GEOMETRY

Example 3: Verifying Triangle Similarity

∆DEF and ∆HJK

Verify that the triangles are similar.

D H by the Definition of Congruent Angles.

Therefore ∆DEF ~ ∆HJK by SAS ~.

GEOMETRY

TEACH! Example 3

Verify that ∆TXU ~ ∆VXW.

TXU VXW by the Vertical Angles Theorem.

Therefore ∆TXU ~ ∆VXW by SAS ~.

GEOMETRY

A A by Reflexive Property of , and B C since they are both right angles.

Example 4: Finding Lengths in Similar Triangles

Explain why ∆ABE ~ ∆ACD, and then find CD.

Step 1 Prove triangles are similar.

Therefore ∆ABE ~ ∆ACD by AA ~.

GEOMETRY

Example 4 Continued

Step 2 Find CD.

Corr. sides are proportional. Seg. Add. Postulate.

Substitute x for CD, 5 for BE, 3 for CB, and 9 for BA.

Cross Products Prop. x(9) = 5(3 + 9)

Simplify. 9x = 60

Divide both sides by 9.

GEOMETRY

TEACH! Example 4

Explain why ∆RSV ~ ∆RTU and then find RT.

Step 1 Prove triangles are similar.

It is given that S T. R R by Reflexive Property of .

Therefore ∆RSV ~ ∆RTU by AA ~.

GEOMETRY

TEACH! Example 4 Continued

Step 2 Find RT.

Corr. sides are proportional.

Substitute RS for 10, 12 for TU, 8 for SV.

Cross Products Prop.

Simplify.

Divide both sides by 8.

RT(8) = 10(12)

8RT = 120

RT = 15

GEOMETRY

Example 5: Writing Proofs with Similar Triangles

Given: 3UT = 5RT and 3VT = 5ST

Prove: ∆UVT ~ ∆RST

GEOMETRY

Statements Reasons

1. Given1. 3UT = 5RT

2. Divide both sides by 3RT.2.

3. Given.3. 3VT = 5ST

4. Divide both sides by3ST.4.

5. Vert. s Thm.5. RTS VTU

6. SAS ~ Steps 2, 4, 56. ∆UVT ~ ∆RST

Example 5 Continued

GEOMETRY

TEACH! Example 5

Given: M is the midpoint of JK. N is the midpoint of KL, and P is the midpoint of JL.

Prove: ∆JKL ~ ∆NPM

GEOMETRY

Statements Reasons

TEACH! Example 5 Continued

1. Given1. M is the mdpt. of JK, N is the mdpt. of KL,

and P is the mdpt. of JL.

2. ∆ Midsegs. Thm2.

3. Div. Prop. of =.3.

4. SSS ~ Step 34. ∆JKL ~ ∆NPM

GEOMETRY

Reflexive, Symmetric, and Transitive Properties of Equality also hold true for similarity of triangles.

GEOMETRY

Lesson Quiz

1. Explain why the triangles are

similar and write a similarity

statement.

2. Explain why the triangles are

similar, then find BE and CD.

GEOMETRY

Lesson Quiz

1. By the Isosc. ∆ Thm., A C, so by the def. of , mC = mA. Thus mC = 70° by subst. By the ∆ Sum Thm., mB = 40°. Apply the Isosc. ∆ Thm. and the ∆ Sum Thm. to ∆PQR. mR = mP = 70°. So by the def. of , A P, and C R. Therefore ∆ABC ~ ∆PQR by AA ~.

2. A A by the Reflex. Prop. of . Since BE || CD, ABE ACD by the Corr. s Post. Therefore ∆ABE ~ ∆ACD by AA ~. BE = 4 and CD = 10.

GEOMETRY 10-2 Proving Triangles Similar; AA, SSS, and SAS Warm Up Warm Up Lesson Presentation Lesson...

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